Possible Clues:

• Superior of a religious community
• Elizabeth behind first letters to Big Sister?
• Religious leader with no need for a highwayman's horse
• Superior nun
• Sailor is married to Elizabeth -- a superior lady
• Person at the top of the order
• Religious leader employing muscle to lay hands on missing pound
• Superior woman?
• Play a leading role at The Abbey overseeing The Three Sisters, among others
• Overseer of sisters